Aging wiring has become a critical issue to the aircraft industry due to recent aircraft incidents. The problem is that over time the insulation on wire becomes brittle and cracks. This exposes the underlying conductive wire, creating the potential for short circuits and fire. Popular methods of monitoring aging wire focus on applying electrical sensing techniques that are sensitive to the conductor’s condition, but not very sensitive to the insulation’s condition. Thus, the development of techniques to quantify and monitor aging wire insulation is highly warranted. Measurement of wire insulation stiffness by ultrasonic guided waves is being tested. Initial laboratory tests were performed on a simple model consisting of a solid cylinder and a solid cylinder with a polymer coating. Experimental measurements showed that the lowest order extensional mode could be sensitive to stiffness changes in the wire insulation. To test this theory, conventional wire samples (typically found in aircraft) were heat damaged in an oven, in a range of heating conditions. The samples were 12, 16 and 20 gage and the heat damage introduced material changes in the wire insulation that made the originally flexible insulation brittle and darker in color. Extensional mode phase velocity increased for the samples that were exposed to heat for a longer duration. Although the heat damage conditions may be more extreme than those encountered in environmental aging, indications are that this technique has the potential to detect and quantify degradation in wire insulation.